DC-DC converter control techniques may include a pulse width modulation (PWM) control in which an output voltage is controlled to be constant by changing a pulse width while maintaining a constant frequency, and a pulse frequency modulation (PFM) control in which an output voltage is controlled to be constant by changing a clock cycle while maintaining a constant pulse width. For example, the PFM control is performed in a mode in which a frequency is changed continuously, or in a mode in which a frequency is changed by excluding a clock used in the PWM control. A switching frequency may be controlled in both modes.
In the PWM control, switching operations are performed per cycle even when a load is high or low. When the load is low, the amount of power consumed for the switching operations is relatively large compared with the amount of power that is supplied, and power supply efficiency may be reduced. Thus, the PWM control may be unsuitable when the load is low. In the PFM control, generally, a frequency range that may be changed (the switching frequency) is not limited. However, when a width in which the frequency is changed increases, broadband radiation noise is generated. For these reasons, a DC-DC converter may be operated in the PWM control when the load is high, and may be operated in the PFM control when the load is low. Japanese Patent Application Laid-Open Publication No. 2003-219637 discusses that frequent switching of driving operations may be reduced by making a difference between a level set when the PWM control is switched to the PFM control and a level set when the PFM control is switched to the PWM control.
A power supply device including an error amplifier may detect an output voltage, perform a switching control based on a difference between the value of the output voltage and the value of a reference voltage, and adjust the output voltage. Operational conditions of such a power supply device may change depending on a ratio between the value of an input voltage and the value of an output voltage. As a result, a difference between the value of an output voltage of an error amplifier and the value of another reference voltage may fail to satisfy given conditions even when an operating mode of the power supply device actually needs to be switched. In this case, it may be difficult to switch the operating mode under desired operating conditions.